Numerical assessment of the analytical models used to determine flexural and shear moduli in I-Beams when the tensile and compressive moduli are different

The different compressive and tensile moduli of fibre reinforced composites have been considered in the analysis of the flexural and shear moduli of I-beams. Firstly, the neutral axis has been determined analytically and then, assuming that location of the neutral axis, the analytical flexural modulus of I-beams has also been obtained. In order to assess the proposed procedure, virtual pure bending and three-point bending tests at different spans have been carried out using the finite element method. The compressive and tensile moduli have been taken into account by defining two parts in the numerical models. The numerical flexural and shear moduli have been determined by reducing the data obtained in the virtual tests. Analytical and numerical results are in good agreement. Therefore, the flexural modulus determined by the proposed analytical approach can be introduced as a material property in the finite element method.     

Effect of internal mold release agent on flexural and inter laminar shear properties of carbon and glass fabric reinforced thermoset composites

The study is focused on thermoset composites reinforced with carbon and glass woven fabrics. Two types of thermoset resins, for example, epoxy and vinyl ester were used as the matrix. Varying concentrations of internal mold releasing (IMR) agent was used in the resin. The composites were cured both at room temperature and at 80°C. The flexural properties were studied using 3‐point bending test method. Further theinter‐laminar shear strength (ILSS) was investigated using the short beam shear strength test based on 3‐point bending. The flexural modulus of room temperature cured epoxy resin is higher than that of high temperature cured epoxy resin and cured vinyl ester resin. The flexural modulus is lowest for 1% IMR sample in epoxy system and the modulus for 0% and 2% epoxy are not significantly different. Lowest flexural strength and modulus can be observed for the combination of reinforcement and curing conditions for samples containing 1% IMR for the epoxy systems. Carbon fiber is found to be less compatible with the vinyl ester resin system and the addition of IMR to the resin degraded the properties further. Inter‐laminar shear strength for epoxy‐based composites is not much affected by presence of IMR, but in case of vinyl ester based composites there is a decrease in ILSS on addition of IMR agent. The study explains variation in flexural properties on addition of IMR and change of curing conditions. These results can be used for ascertaining variation in mechanical properties in real use.     

Changes in the mechanical properties of tooth-colored direct restorative materials in relation to time

The objective of this study was to determine the flexural strength, flexural modulus, Vickers hardness of a packable composite (Surefil), and an ormocer (Definite) in comparison with a microhybrid composite (Z-100), a microfil composite (Silux Plus) and a polyacid-modified composite resin (Dyract). Flexural strength and flexural modulus were determined using a three-point bending device. Microhardness was measured with a Vickers indentor. The specimens of each material were prepared according to manufacturer's instructions. The specimens were stored in artificial saliva at pH 6, all at 37°C. The groups were tested at the beginning of the test, at 3 months and at 6 months. Flexural strength values of Surefil and Definite showed a progressive increase. The highest MPa values were determined for Surefil (134.4 MPa) and the lowest MPa values were obtained for Dyract (59.6 MPa). The highest flexural modulus values were revealed for Surefil (10.000 GPa). Z-100, Silux Plus and Definite showed a tendency to decline in relation to time for their flexural modulus. GPa values of Silux Plus were stable at 3 and 6 months. Vickers hardness numbers showed that Surefil was the hardest and Dyract was the weakest material. Copyright © 2003 John Wiley & Sons, Ltd.     

Analysis of a reversible five-point bending configuration based on a novel two-sense support

A novel two-sense support for flexural tests has been designed and manufactured in Ikerlan. The aim of this support is to do two-sense bending fatigue tests. In order to reduce the displacement corresponding to a given stress, a novel test configuration, designated as five-point bending, is modelled analytically. Basically, it is a three-point configuration with two supports at the ends that exert forces in the same sense as the applied load. In this way, a partial clamping is obtained that can be modelled by concentrated loads. The model has been checked carrying out quasi-static three-point and five-point bending tests at different spans in unidirectional carbon/epoxy composite specimens. Flexural modulus and the out-of-plane shear modulus have been obtained by linear regression in both cases, after having obtained experimentally the stiffness of the system.     

动态固化聚丙烯/环氧树脂共混物的研究

将动态硫化技术应用于热塑性树脂 热固性树脂体系 ,制备了动态固化聚丙烯 (PP) 环氧树脂共混物 .研究了动态固化PP 环氧树脂共混物中两组分的相容性、力学性能、热性能和动态力学性能 .实验结果表明 ,马来酸酐接枝的聚丙烯 (PP g MAH)作为PP和环氧树脂体系的增容剂 ,使分散相环氧树脂颗粒变细 ,增加了两组分的界面作用力 ,改善了共混物的力学性能 .与PP相比 ,动态固化PP 环氧树脂共混物具有较高的强度和模量 ,含 5 %环氧树脂的共混物拉伸强度和弯曲模量分别提高了 30 %和 5 0 % ,冲击强度增加了 15 % ,但断裂伸长率却明显降低 .继续增加环氧树脂的含量 ,共混物的拉伸强度和弯曲模量增加缓慢 ,冲击强度无明显变化 ,断裂伸长率进一步降低 .动态力学性能分析 (DMTA)表明动态固化PP 环氧树脂共混物是两相结构 ,具有较高的储能模量 (E′)     

Mechanical,low-velocity impact,and hydrothermal aging properties of flax/carbon hybrid composite plates

A hybrid of flax and carbon fibers was considered as an effective way to enhance the mechanical and hydrothermal resistance of flax-reinforced polymer composites. In this study, hybrid composites based on three layers of cross-ply flax fabrics, two layers of unidirectional carbon fabrics, and an epoxy resin were investigated in terms of the tensile, three-point bending, impact, and water absorption properties. The flax fabric reinforcement of the hybrid composites contributed to an improvement in the toughness, whereas the carbon fabric contributed to an improvement in their hydrothermal resistance and overall strength and stiffness. The hybrid composites with carbon fibers on the surface (CFFFC) exhibited brittle failure in the tensile test, whereas those with alternating layers (FCFCF) exhibited greater plastic deformation. In addition, the failure strain of the CFFFC samples showed a negative hybrid effect, whereas that of the FCFCF samples improved 63.5% compared with that of carbon-fiber-reinforced polymer composites. A positive hybrid effect on the impact performance of hybrid reinforced epoxy composites containing the unidirectional carbon fabric and cross-ply flax fabric was observed. At 40 °C and 80% relative humidity, the diffusion rate of water molecules in the FCFCF samples was 16 times that in the CFFFC samples.     

Preparation and characterization of functionalized graphene oxide/carbon fiber/epoxy nanocomposites

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy adhesive, with the aim of improving the bonding strength of carbon fiber/epoxy composite. The chemical structure of the functionalized GO (FGO) nanosheets was characterized by elemental analysis, FT-IR and XRD. Hand lay-up, as a simple method, was applied for 3-ply composite fabrication. In the sample preparation, the fiber-to-resin ratio of 40:60 (w:w) and fiber orientations of 0°, 90°, and 0° were used. The GO and FGO nanoparticles were first dispersed in the epoxy resin, and then the GO and FGO reinforced epoxy (GO- or FGO-epoxy) were directly introduced into the carbon fiber layers to improve the mechanical properties. The GO and FGO contents varied in the range of 0.1–0.5 wt%. Results showed that the mechanical properties, in terms of tensile and flexural properties, were mainly dependent on the type of GO functionalization followed by the percentage of modified GO. As a result, both the tensile and flexural strengths are effectively enhanced by the FGOs addition. The tensile and flexural moduli are also increased by the FGO filling in the epoxy resin due to the excellent elastic modulus of FGO. The optimal FGO content for effectively improving the overall composite mechanical performance was found to be 0.3 wt%. Scanning electron microscopy (SEM) revealed that the failure mechanism of carbon fibers pulled out from the epoxy matrix contributed to the enhancement of the mechanical performance of the epoxy. These results show that diamine FGOs can strengthen the interfacial bonding between the carbon fibers and the epoxy adhesive.     

Preparation and Characterization of DGEBA/EPN Epoxy Blends with Improved Fracture Toughness

The physical and mechanical properties of blends composed of two kinds of epoxy resins of different numbers of functional groups and chemical structure were studied.One of the resins was a bifunctional epoxy resin based on diglycidyl ether ofbisphenol A and the other resin was a multifunctional epoxy novolac resin.Attempt was made to establish a correlation between the structure and the final properties of cured epoxy samples.The blend samples containing high fraction of multifunctional epoxy resin showed higher solvent resistance and lower flexural modulus compared with the blends containing high fraction of bifunctional epoxy resin.The epoxy blends showed significantly higher ductility under bending test than the neat epoxy samples.The compressive modulus and strength increased with increasing of multifunctional epoxy in the samples,probably due to enhanced cross-link density and molecular weight.Morphological analysis revealed the presence of inhomogeneous sub-micrometer structures in all samples.The epoxy blends exhibited significantly higher fracture toughness (by 23％ at most) compared with the neat samples.The improvement of the fracture toughness was attributed to the stick-slip mechanism for crack growth and activation of shear yielding and plastic deformation around the crack growth trajectories for samples with higher content of bifunctional epoxy resin as evidenced by fractography study.     

SYNTHESIS, CHARACTERIZATION AND PROPERTIES OF ORGANOCLAY-MODIFIED POLYSULFONE/EPOXY INTERPENETRATING POLYMER NETWORK NANOCOMPOSITES

Organoclay-modified hydroxylterminated polysulfone (PSF)/epoxy interpenetrating network nanocomposites (oM-PSF/EP nanocomposites) were prepared by adding organophilic montmorillonite (oMMT) to interpenetrating polymer networks (IPNs) of polysulfone and epoxy resin (PSF/EP) using diaminodiphenylmethane (DDM) as curing agent.The mechanical properties like tensile strength,tensile modulus,flexural strength,flexural modulus and impact properties of the nanocomposites were studied as per ASTM standards.Differ...     

Uniaxial tensile tests and dynamic mechanical analysis of satin weave reinforced epoxy shape memory polymer composite

The utilization of epoxy shape memory polymer composite (SMPCs) as engineering materials for deployable structures has attracted considerable attention in recent decades due to high strength and satisfactory stiffness in comparison with shape memory polymers (SMPs). Knowledge of static and dynamic mechanical properties is essential for analyzing structural behavior and recovery properties, especially for new epoxy SMPCs. In this paper, a new weave reinforced epoxy shape memory polymer composite was prepared with satin weave technique and resin transfer molding technique. Uniaxial tensile tests and dynamic mechanical analysis were carried out to obtain basic mechanical properties and glass transition temperatures, respectively.The tensile strength and breaking elongation of warp specimens were comparable with those of weft specimens. The increment of elastic modulus and hysteresis loop areas became smaller with loading cycles, meaning that cyclic tests could obtain approximate stable mechanical properties. For dynamic mechanical properties, glass transition temperature (T_g) obtained from storage modulus curves was lower than that determined from tan delta curves and T_gs in the warp and weft directions were similar (29.4 °C vs 29.7 °C). Moreover, the storage modulus in response to T_g was two orders of magnitude less than that with respect to low temperature, which demonstrated the easy processibility of epoxy SMPCs near glass transition temperature. In general, this study could provide useful observations and basic mechanical properties of new epoxy SMPCs.     

Investigation of flexural properties and failure behaviour of biaxial braided CFRP

In this research, the experimental tests of quasi-static three-point bending and three-point bending fatigue were carried out for a ±25° biaxial braided carbon fibre reinforced polymer (CFRP) manufactured using vacuum assisted resin transfer moulding (VARTM). A finite element (FE) model was also set up for quasi-static testing and the prediction results revealed that local fibre volume fraction (FVF) is a primary source affecting the mechanical properties of braided CFRP. The fatigue of the braided CFRP was defined as three different stages according to the flexural modulus results. The damage modes of the test specimens were observed via a digital microscope and scanning electron microscope (SEM) and the process-induced defects were summarised. With compiled results and observations, this study provides a better understanding of failure and fatigue behaviour of biaxial braided composites and their flexural properties which offers a good basis for any further research in fibre volume fractions, structure design and manufacturing for braided CFRP.     

Determination of longitudinal compressive strength of long fiber composites by three-point bending of [0m/90n/0p] cross-ply laminated strips

A new test method is proposed for measuring longitudinal compressive strength of composite laminates by three-point bending of cross-ply laminates. Optimal cross-ply configuration has been designed in order to get compressive stresses higher than tensile stresses. Thermal and mechanical stresses have been calculated based on the hypothesis of Classical Beam Theory. Cross-ply carbon/epoxy strips with different thicknesses and spans have been tested by three-point bending. Failures on the compressive side have been observed in all cases and no evidence of transverse cracking has been found. Experimental results agree well with compressive strength reference values. The possibility of determination of compressive and tensile strengths by three-point bending from a unique cross-ply lay-up has been also analyzed.     

Epoxy nanocomposites co-reinforced by two dimensionally different nanoscale particles

Comprehensive high-performance epoxy nanocomposites were prepared by simultaneous incorporating montmorillonite (MMT) and nanoSiO₂ into epoxy. Mechanical tests and thermal analyses showed that the epoxy/MMT/nanoSiO₂ nanocomposites obtained considerable improvement over basic epoxy in tensile modulus, tensile strength, flexural modulus, flexural strength, notch impact strength, glass transition temperature, and thermal decomposition temperature. X-ray diffraction measurements and transmission electronic microscopy observations revealed that the layered structure of MMT was completely exfoliated into two-dimensional nanoscale mono-platelets. These 2D mono-platelets formed intermingled structure with the zero-dimensional nanoSiO₂ spheres in the nanocomposites. This study suggests that employing the synergistic reinforcement effect of two dimensionally different nanoscale particles is one pathway to success in developing comprehensive high-performance polymer nanocomposites.     

Preparation and characterization of bismaleimide/1,3-dicyanatobenzene modified epoxy intercrosslinked matrices

An intercrosslinked network of cyanate ester (CE)-bismaleimide (BMI) modified epoxy matrix system was made by using epoxy resin, 1,3-dicyanatobenzene and bismaleimide (N,N^′-bismaleimido-4,4^′-diphenyl methane) with diaminodiphenylmethane as curing agent. BMI-CE-epoxy matrices were characterised using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and heat deflection temperature (HDT) analysis. The matrices, in the form of castings, were characterised for their mechanical properties such as tensile strength, flexural strength and unnotched Izod impact test as per ASTM methods. Mechanical studies indicated that the introduction of cyanate ester into epoxy resin improves the toughness and flexural strength with reduction in tensile strength and glass transition temperature, whereas the incorporation of bismaleimide into epoxy resin influences the mechanical and thermal properties according to its percentage content. DSC thermograms of cyanate ester as well as BMI modified epoxy resin show an unimodal reaction exotherm. Electrical properties were studied as per ASTM method and the morphology of the BMI modified epoxy and CE-epoxy systems were studied by scanning electron microscope.     

The curing kinetics and mechanical properties of epoxy resin composites reinforced by PEEK microparticles

In this paper, a polyether-ether-ketone (PEEK)/epoxy composite was prepared by using PEEK microparticles as the reinforcement. The nonisothermal differential scanning calorimetry (DSC) test was used to evaluate the curing reaction of PEEK/epoxy resin system. The curing kinetics of this system were examined utilizing nonisothermal kinetic analyses (Kissinger and Ozawa), isoconversional methods (Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose) and an autocatalytic reaction model. During these analyses, the kinetic parameters and models were obtained, the curing behavior of PEEK/epoxy resin system under dynamic conditions was predicted. The results show that isoconversional methods can adequately interpret the curing behavior of PEEK/epoxy resin system and that the theoretical DSC curves calculated by the autocatalytic reaction model are in good agreement with experimental data. Furthermore, the tensile elongation at break, tensile strength, flexural strength, compression strength and compression modulus increased by 81.6%, 33.66%, 36.53%, 10.98% and 15.14%, respectively, when PEEK microparticles were added in epoxy resin composites.     

聚醚链段长度对氨基聚醚-环氧树脂力学性能的影响

以柔性端氨基聚醚(BATPE)和双酚A环氧树脂(DGEBA)为原料, 制备了无微相分离结构的无定型AB交联热固性树脂. 测试了3种不同聚乙二醇(PEG)链段长度(M_PE)的BATPE-DGEBA环氧树脂固化产物的应力-应变曲线、动态力学温度谱和冲击断面形貌. 结果表明, 在环氧树脂交联网络中引入两端与DGEBA化学连接的PEG链段能避免微相分离结构的生成, 有利于提高DGEBA链段的应变松弛速率. 增加M_PE, 一方面能降低环氧树脂固化产物的玻璃化转变温度和室温下的刚度和拉伸强度, 增加韧性(包括冲击强度和拉伸韧性)、断裂应变和模量损耗因子; 另一方面也能提高固化产物在低温下的储存模量. 优化M_PE可制备出在中低温下同时具有优异的拉伸强度、模量、断裂应变和冲击性能的BATPE-DGEBA环氧树脂.     

Dynamic mechanical properties of glass-filled epoxy resin

Composite specimens were prepared using soda glass beads and a purified epoxy resin cured with 1,3-propylene diamine. Some beads were treated with a silane coupling agent. The dynamic mechanical properties of these specimens were measured in the temperature range ?190 to +180°C using a free-oscillation torsion pendulum. The dynamic mechanical relaxation spectrum showed no feature that could be attributed to the formation of a new interfacial phase and the torsional moduli were unaffected by the use of the coupling agent. Increasing the glass content of the specimens decreased the damping and increased the modulus. An attempt was made to predict the composite modulus using the Kerner equation. When the specimens were immersed in boiling water, two effects were noted. First, water was absorbed in the epoxy resin matrix and changes in the dynamic spectrum were observed. Second, in samples filled with untreated glass debonding occurred and the presence of free water at the interface was indicated by the appearance of a new peak near 0°C.     

Measurement of flexural modulus of polymeric foam with improved accuracy using moiré method

The flexural modulus of polymeric foams determined from three-point bending tests is usually inaccurate due to the local deformation undergone by the material during testing. The machine used in the test gives deflection values larger than the actual deflection of the foam specimen due to the deformation of the material at the loading point. This leads to errors in the computation of the modulus value. In this work, the deflection values of a beam made of polymeric foam in a three-point bending test were determined using the moiré method. The change in the moiré pattern at the neutral axis of the foam during loading was recorded and converted into deflection values. The deflection data were used to generate the stress–strain curve from which the flexural modulus of the foam material was determined. The proposed method was verified using aluminum beams, where a high correlation between the deflection data from the machine readings and the moiré method was obtained. The flexural modulus of the foam determined using the moiré method was found to be within 3% of the value published in the material data sheet.     

Effects of Wood Flour (WF) Pretreatment and the Addition of a Toughening Agent on the Properties of FDM 3D-Printed WF/Poly(lactic acid) Biocomposites

In order to improve the properties of wood flour (WF)/poly(lactic acid) (PLA) 3D-printed composites, WF was treated with a silane coupling agent (KH550) and acetic anhydride (Ac₂O), respectively. The effects of WF modification and the addition of acrylicester resin (ACR) as a toughening agent on the flowability of WF/PLA composite filament and the mechanical, thermal, dynamic mechanical thermal and water absorption properties of fused deposition modeling (FDM) 3D-printed WF/PLA specimens were investigated. The results indicated that the melt index (MI) of the specimens decreased after WF pretreatment or the addition of ACR, while the die swell ratio increased; KH550-modified WF/PLA had greater tensile strength, tensile modulus and impact strength, while Ac₂O-modified WF/PLA had greater tensile modulus, flexural strength, flexural modulus and impact strength than unmodified WF/PLA; after the addition of ACR, all the strengths and moduli of WF/PLA could be improved; after WF pretreatment or the addition of ACR, the thermal decomposition temperature, storage modulus and glass transition temperature of WF/PLA were all increased, and water absorption was reduced.     

Epoxy shape memory polymer (SMP): Material preparation,uniaxial tensile tests and dynamic mechanical analysis

The utilization of epoxy shape memory polymers (SMPs) as engineering materials for deployable structures has attracted considerable attention due to their excellent thermo-mechanical endurance and satisfactory processability. Knowledge of static and dynamic mechanical properties is essential for analyzing structural behavior and recovery, especially for new epoxy SMPs. In this paper, a new epoxy SMP was prepared with epoxy and aromatic amine curing agent. Uniaxial tensile tests and digital image correlation were used to obtain static mechanical properties. Dynamic mechanical analysis was carried out to evaluate glass transition temperatures that corresponded to the heat in the recovery process.It was found that elastic modulus, Poisson’s ratio and shear modulus are 1413 MPa, 0.35 and 591 MPa, respectively. The beginning of glass transition temperature of 37.4 °C could be effectively achieved by electrical heaters, validating the shape memory properties of epoxy SMPs. In general, this study could provide useful observations and basic mechanical properties of epoxy SMPs.